1. Field of the Invention:
The present invention relates to image projection systems. More specifically, the present invention relates to systems for projecting electronically described dynamic imagery on a large screen.
2. Description of the Related Art:
Many applications require electronically described images to be projected on large screens, with high brightness, high resolution, and full color. Several technologies utilized for this purpose are currently known in the art including: 1) cathode-ray tubes, 2) active matrix liquid crystal light valves, 3) photo-activated liquid crystal light values and 4) light valves that work by deflecting portions of the light falling onto them, such as the Digital Micromirror Device.RTM..
Cathode ray tube devices are best known and most prevalent. These devices are used for numerous applications including television displays and computer monitors. A cathode ray tube can be described as consisting of an electron gun to produce a beam of electrons, focus and deflection circuitry to paint the electrons onto a series of points on its faceplate, and a phosphorescent faceplate screen. The impact of the electrons onto the molecules of the phosphor in the screen generates photons. An image is formed by electronically controlling how many electrons land at each point on the screen over a given period of time.
For those color applications where the screen can be viewed directly, three electron guns are generally packaged within one cathode ray tube in a manner such that the electrons from each gun impact only the phosphor of the color associated with its contribution to the image, red, green or blue. For those applications requiring large luminous flux to achieve high brightness on a large screen, three cathode-ray tubes are generally used so as to maximize the available brightness. Unfortunately, there is still a limit to the maximum brightness attainable as the intensity of the electron beams cannot be increased past the point where the phosphor screens are damaged.
Light valve projectors use a spatial light modulator to impart spatial and temporal modulation to light from a high intensity source. In a liquid-crystal light valve, the electrical voltage applied across the liquid crystal material is generally used to modulate the polarization of the optical wavefront from the light source, and by subsequently passing the modulated light through another polarizer, often called an analyzer, one can obtain a light beam whose intensity is related to the applied electrical voltage. In other light valves, tilting mirrors or other mechanical means are used to control whether or not the light from the lamp passes through an aperture stop and onto the screen.
In liquid crystal light valves, the electrical voltage applied across a thin film of liquid crystal material is modulated spatially and temporally so as change the optical properties of the liquid crystal material as a function of its location at any given instant in time. In an active-matrix liquid-crystal light value, the most prevalent type of liquid-crystal display at the present time, row and column electrodes are used to channel the electrical signals to the appropriate location at the desired point in time. In a photo-activated light valve (sometimes known as an Image Light Amplifier), a device expressly designed for projection applications, the image on the phosphor screen of a CRT is reimaged onto a photo-conductor which in turn controls the electrical voltage applied across the film of liquid crystal material. Unfortunately, liquid-crystal light valve projectors are complex to manufacture, and the analog nature of the light modulation process makes it difficult to achieve high spatial and temporal uniformity.
Several groups have proposed using a micromachined device built on a silicon integrated circuit as a light valve. One example of such a device is the Digital Micromirror Device.RTM. (DMD). In one embodiment the DMD consists of a complementary metal-oxide semiconductor (CMOS) static random access memory (RAM) chip with an array of mirrors mounted over the surface of the chip such that there is a one-to-one relationship between each memory cell and a mirror. Each mirror has a deformable mount such that it can be tilted to one of two stable positions depending on the data stored in the corresponding memory cell. In the "on" position, for example, a mirror is tilted to allow light incident on the array to pass through a tiny aperture for projection onto a screen. In the "off" position, the mirror is not tilted, and incident light is reflected away from the projection aperture. Hence, by programming the tilt of each mirror in the array of mirrors as a function of time, spatial and temporal modulation may be imparted to the otherwise uniform illumination from the light source. With a suitable lens, the light reflected by the array of mirrors may be focused onto a screen for viewing.
Like other devices of its class, the DMD is digital, with each cell being either "on" or "off". Thus, some systems had to be developed to produce the gray scale required to create quality pictorial images. Gray scale image projection in these devices is effected currently by varying the amount of time that the mirror is tilted. In particular, the DMD currently uses a bit sequential method of displaying gray scale. The data for the most significant bit is displayed by tilting the mirror for 1/2 of the total frame time, the second most significant bit is displayed for 1/4 the total frame time, and so on. Thus, for a system which uses an eight bit digital word to display video data at each pixel, if each bit is written immediately before it is displayed, the time available to write each bit nominally varies from 1/2 frame to 1/256 or 1/(2 8)! of a frame, assuring 8-bit video which gives you seven steps in addition to fully "on" and "off".
In a recent article by Claud Tew et al. entitled "Electronic Control of a Digital Micromirror Device.RTM. for Projection Displays," published in the 1994 IEEE International Solid-State Circuits Conference, pp. 130-131, at least two different designs are described for Digital Micromirror Devices.RTM.. However, both of these involve pulse-width modulation implemented by rapidly flipping the mirrors back and forth to achieve gray scale images, images that appear to have regions whose brightness is between black and white. Image brightness is controlled by the period of time during which the mirror for a given unit cell or pixel is in the "on" position. In an eight bit system implementation, the shortest field period required is nominally 1/256 times as long as the frame period. During this time the system must be able to write the data for the next bit field (otherwise the DMD will not be ready to change state when required). With a typical frame rate of 1/60th of a second (consistent with US TV standards), the dimmest bit can be on for no more than 1/256 times 1/60th of a second. This presents speed problems which are exacerbated in larger arrays where perhaps 1080 lines of 1920 pixels (a proposed HDTV standard) must be addressed during each field. Color can be presented by using three DMDs, on for each primary color, or by using one device presenting the three colors sequentially. The later approach is less costly and thus more desirable, but it further exacerbates the speed problem by requiring everything to run three times faster.
Thus, there is a need in the art for an inexpensive system and technique for displaying electronically described images, possibly with high brightness (e.g. 5000 lumens), on a large screen. In particular, there is a need in the art for an inexpensive system and technique for displaying electronic images of varying intensity in color.